CCBC-534: Internal v2 wrappers should use lcb_sched_leave()

This enables lcb_sched_flush and LCB_CNTL_IMPLICIT_FLUSH to work
properly.

Change-Id: I6a2c703f823cb7a9f7ac81457ae178b64ca898fd
Reviewed-on: http://review.couchbase.org/42613
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>